Various theories as to the shapes and rela tive volumes of the different atoms exist, some of which assume close packing, others the exist ence of interstices, some base the volume on the atomic weight, others on the valence, and each succeeds in explaining many peculiarities of structure.
Experimental Proof of the Existence of a Regular Interhal Structure.— Light rays are diffracted by ruled when the inter vals between the parallel rulings are of the order of the wave length of light used. Thus sodium light with wave length of .000589 milli meters is diffracted by a ruling of 700 lines per millimeters, that is, with intervals of .00143 millimeters.
Dr. Lane of Zurich conceived the idea of using a crystal as a diffraction grating for X-rays, the waves of which were believed to be very much shorter than those of light and in 1912 by passing a pencil of X-rays through a crystal plate obtained upon a photographic plate not only a strong spot due to the original pencil but a series of other isolated spots which were always arranged in conformity with the sym metry of thecrystal and evidently due to re flection from different sets of net planes.
W. L. Bragg showed that for any set of parallel net planes the reflections were obtained only for particular angles of incidence which corresponded to particular intervals between successive planes such that the crests of the reflected waves from each plane agreed, that is, to intervals of 1, Z 3, etc., wave lengths, where as for other intervals the different reflected waves tended to neutralize each other. Hence it followed that from the angles and the wave lengths the intervals could be calculated.
The device for measuring these intervals is called an X-ray spectral photometer. A fine pencil of X-rays is directed upon the crystal and the latter is revolved, at certain angles a pencil of rays is reflected into an ionization chamber and its presence being made known by the connected electroscope, the angle is re corded.
Variations in intensity indicate furthermore variation in spacing between similar net planes or lack of similarity between net planes uni formly spaced.
Crystal Growth.—Many strange views have been held as to the causes of crystallization, such as °intense cold,* °divine fire,* °the influence of the stars from which the six-rayed snow as tals come, 1) °the influence of a particular and the emotions of organic molecules, espe cially those derived from the remains of ani mals and plants?' Later theories involved the idea of an em bryonic period during which the forming crys tal lacked characteristic shape. This was based
on the study of the crystallization of sulphur from solution in carbon disulphide during which there were observed tiny globules called °glob which according to Vogelsang's the ory are liquid spheroids or ellipsoids with a definite internal molecular movement, which renders them capable of mutual attraction. These group themselves in definite forms, some times in rows, like strings of beads, amargar ites,D and again in needle-like or conical forms, °longulites.) From these develop °crystalloids) of characteristic shape, often doubly refracting, and so on to the complete crystal.
It was found, however, that the globules were carbon disulphide supersaturated with sul phur and prevented from crystallizing by the t viscous Canada balsam and moreover from pure solutions the minutest crystals are the same shapes as the larger ones, as, for instance, when very minute, perfectly formed crystals are obtained by suddenly diminishing the solu bility by the addition of another liquid.
It appears from the researches of Miers that there are two distinct varieties of crystalliza tion possible in any cooling fused mass or any evaporating solution, °labile) or spontaneous crystallization and ametastableD or induced crys tallization requiring the presence of °germs' that is, minute crystals of the substance or a related substance. In studying the phenomena he made use of the fact that the refractive in dices of solution or fusions varied directly as the saturation and were highest just preceding spontaneous crystallization.
Labile crystallization occurs only with super saturation or undercooling but in the interval between this and normal saturation or the melt ing temperature crystals will form by inocula tion of the mass with crystal ugerms.D This has been experimentally proved both by induc ing crystallization by stirring, in an open vessel and by failure to induce crystallization either if the substance was new and its germs were not in the room or if the air was excluded as in a sealed tube. In all instances, however, °labile' crystallization occurred.